1. Field of the Invention
The present invention relates to catheter type blood pressure measuring devices, which use transducers to develop signals responsive to fluid pressure waves present in blood vessels in which the catheters are inserted; and presents an apparatus for damping out resonant pressure waves from the pressure waves truly representative of the blood pressure in the vessel.
2. Description of the Prior Art
It is well known in the prior art that blood pressure can be measured by means of a catheter inserted in a patient's vessel. See, for example, U.S. Pat. No. 2,600,324 to Rappaport issued June 10, 1952 and U.S. Pat. No. 3,122,136 to Murphy issued Feb. 25, 1964.
The damping of pulses in fluid is also known in the prior art. An example of damping is shown in U.S. Pat. No. 2,064,268 to Parker issued Dec. 15, 1936, in which movable rods in a pair of cylinders are used to absorb some of the fluid pressure. This device was not related to sterile medical applications, but does show one method of damping of fluid pressure.
It is known in the prior art that reducing the size of the opening through which fluid waves move will reduce net pressure. In patents, such as U.S. Pat. No. 1,121,105 to Karlson issued Dec. 15, 1914, this method was used to prevent surges of pressure from breaking the old-fashioned reading devices.
It is known that resonant waves can reduce the accuracy of the reading of blood pressure in catheter-type measuring devices. U.S. Pat. No. 3,865,100 to Kanai et al issued Feb. 11, 1975 attempted to solve this through a resilient damping device attached to a transducer which absorbs some of the pressure. This Kanai et al patent teaches no method of adjusting the rate of damping.
The following patents were found in a search of an early form of the invention; but do not seem particularly relevant to the present invention:
______________________________________ U.S. PAT. DOCUMENTS U.S. PAT. NO. INVENTOR ISSUE DATE ______________________________________ 1,905,782 Amdursky April 25, 1933 2,841,984 Green July 8, 1958 3,157,201 Littmann November 17, 1973 3,769,964 Smith March 6, 1973 4,072,146 Howes February 7, 1978 4,206,761 Cosman June 10, 1980 ______________________________________
The following four publications discuss methods of correcting blood pressure measurement. An article entitled "Continuous Flushing Systems" by R. D. Latimer and K. E. Latimer in "Anesthesia", 1974, Volume 29, pages 307-317, discusses adding inertive resistance to the catheter system, but primarily focuses on parallel damping. It does not teach series damping by restriction.
An article entitled "Long-Term Clinical Recording of Static and Dynamic Intra-Arterial Blood Pressure" by B. Magnes and H. Nornes, Acta anaesth, Scand. 1974, 18, 215-223, discusses the use of a screw clamp to compress the infusion tubing. This article does not address the problem of resonant waves, however. This refers to the outdated problem of increased compliance in the monitoring system introduced by what are now-obsolete infusion machines. This screw clamp method isolated the infusion machine from the monitoring system and thus eliminated the extra compliance which it introduced. The problem addressed by the present invention is that of too little compliance or too little friction, which is the opposite of what is taught by this reference.
A book by L. A. Geddes entitled "The Direct and Indirect Measurement of Blood Pressure" discusses in Chapter 1, pages 9 through 69, the relation between the length of a needle for blood pressure and the damping coefficient. This article in general discussed the proper damping coefficient to be achieved by a system for measuring blood pressure. This article arrived at 0.7 as a theoretical damping coefficient needed to obtain critical damping. After this article, it has been found in practical experiments that 0.64 is the optimum practical value.
In a book entitled "Cardiovascular Fluid Dynamics" by D. H. Bergel from Academic Press 1972, Chapter 2 by I. T. Gabe at pages 11 through 50, includes an early discussion of damping waves in blood pressure measuring devices. This article discusses an older technology, such as placing a damping resistance at the open end of the catheter. This is unsatisfactory in that it is within the patient and inaccessible to external adjustment. The article also mentions the possibility of varying damping resistance with a needle valve. No apparatus for accomplishing this is known. The prior art encountered the problem of damping devices preventing flushing of the catheter. If the apparatus is not constantly flushed, blood will clot in it and block the system. This long-felt problem was not resolved in the prior art. After this mentioned of series damping by restriction, the article goes on to discuss parallel damping, which is the method followed by the industry every since. Parallel damping involves adding resistance and compliance to the measuring system out of line with the flow of pressure waves. Parallel damping is inferior to damping in series for two reasons. First, parallel damping often uses an air cushion to absorb pressure. This air may escape into the system. Air in the system can distort pressure readings and can also harm or kill the patient if it escapes into a blood vessel. Second, the amount of damping effect achievable is less with a parallel system. The more severe cases of resonance cannot be controlled as effectively by a parallel device.
No apparatus has been found in the prior art which provides variable series damping. No such apparatus has solved the problem of interference with constant flushing of the system.